Recently amplified interannual variability of the winter severity and ice cover over the North American Great Lakes in response to changing teleconnections

The North American Great Lakes (hereafter, the Great Lakes) are the largest world's freshwater system and are partially frozen each year. While the Great Lakes annual maximum ice cover (AMIC) has decreased in recent decades in response to global warming, notable year-to-year fluctuations exist, and even in recent years, extreme cold winters occurred (e.g., the winter of 2018/2019). In order to understand how the occurrences of these extreme winters are connected to large-scale atmospheric circulations, we conduct statistical and process-oriented analyses from 1980 to 2020 using the time series of AMIC, atmospheric reanalysis datasets, and the North Pacific sea surface temperature (SST) data. We identify a reduced number of accumulated freezing degree days across the winter months in recent decades, a step-change decrease of AMIC after the winter of 1997/98, and an increased interannual variability of AMIC since 1993. AMIC is significantly correlated with El Niño-Southern Oscillation, the North Atlantic Oscillation, and the Pacific/North American pattern before the winter of 1997/98. After that, the AMIC is significantly correlated with the Tropical/Northern Hemisphere pattern and Eastern Pacific Oscillation. Singular value decomposition of the 500 hPa geopotential height and surface air temperature shows a dipole pattern over the northeast Pacific and North America, demonstrating the ridge-trough system. This dipole pattern shifts northward after 1997/98, which coincides with the change in SST over the northeast Pacific, where the second mode of the empirical orthogonal function (EOF) on SST shows a warm blob-like feature manifested over the Gulf of Alaska. The regression of wave activity flux onto the SST EOF shows that the source of upward and eastward propagation of a stationary Rossby wave shifts to the west coast of North America, likely moving the ridge-trough system eastward after the winter of 1997/98 to place the Great Lakes region in the trough.